In general, “full duplex” communication devices, such as full duplex telephones or headsets, can simultaneously transmit and receive audio signals, thereby allowing a user to simultaneously talk and listen to another party through a speakerphone or separate microphone and speaker of the device. With full duplex telephony devices, however, audio signals may be distorted with “echo” signals that are generated due to acoustical and/or electrical feedback. For example, line echo may be generated when some portion of a near end party's speech signals are coupled from the speakerphone microphone circuit to the speaker circuit. Line echo signals are generated due to reflection caused by impedance mismatches along the transmit and receive paths (e.g., due to imperfect impedance match at four-wire to two-wire conversion points, known as hybrids). Another type of echo caused by acoustic feedback is where acoustic coupling occurs between the speaker and the microphone. Acoustic energy produced by the speaker may reflect from nearby objects or be coupled directly to the microphone. This feedback produces a phase delay in the coupled audio signal making a hollow sounding echo.
Moreover, with full duplex speakerphone devices, the microphone output level can affect signal quality. For instance, a microphone output level may be too low when a near end person is talking at a distance of more than 6 feet on the phone. This situation produces a “hollow” or “in a tunnel” sound effect to a listener on the far end of the conversation. Moreover, a microphone output level may be too high when, e.g., when a near end person is talking loud or at a distance of two feet or less. This situation produces excess sidetone within the system,
To eliminate or reduce the degraded audio quality due to echoes, full duplex communication devices typically implement various types of acoustic digital signal processing techniques known in the art. For example, Line Echo Cancellation is used to remove relatively short delay components of telephone network line echo (e.g. network Side-Tone) from the incoming receive signal. However, depending on the dynamic range for such digital circuits, performance can be affected by microphone output levels. Indeed, the dynamic range of the microphone output level is typically greater than the dynamic range of the digital signal processing circuitry, e.g., digital Line Echo Cancellation chip. For example, a microphone output level may have a dynamic range of 60 dB, whereas a digital Line Echo Cancellation chip can effectively handle a dynamic range of about 45 to 50 dB.
One conventional solution to limit the dynamic range is to implement digital signal processing functions, such as companding functions, after digitizing the input. However, such solutions are not effective if the dynamic range of the input exceeds the range of the input ND conversion, which adversely affects the echo-canceling capabilities. Another solution is to adjust the microphone output level to minimize one problem or the other, or pick a mid point, which yields both problems.